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Geologic Dating!
Interpreting Fossil Evidence
• The natural forces that form sedimentary rock can also
reveal fossils that have been hidden in layers of rock for
millions of years.
• As forces within the earth uplift rock and weathering forces
wear away rock, fossils become exposed.
• Remember: A fossil is evidence (remains or traces) of an
organism that once lived.
• A paleontologist is a scientist that studies fossils and the
environments in which the organism lived.
• Scientists try to determine its age and anatomical
similarities and differences to living organisms.
Relative Dating
• Law of Superposition – newer layers of rock/soil are on top of older layers. The
lowest layer is the oldest, the top most layer, the most recent (youngest). These
are called “strata”.
• Stratification: enables a scientist to establish the “relative age” of something
based on where it is found compared to other layers.
• Relative Dating – estimating the age of a fossil by comparing its placement with
that of fossils in other layers of rock to “determine the sequence in which events
occurred, not how long ago they occurred.”
– Problem: This dating provides no information of absolute age or age in years.
– Question: Where will the oldest fossils be located? On the top or on the bottom?
Using Fossils to Date Rocks
• The principle of Fossil succession
states that fossil organisms succeed
one another in a definite and
determinable order. Therefore, any
time period can be recognized by
its fossil content.
• Index Fossils – fossils of organisms
that lived during a relatively short,
well-defined time period.
– Ammonites (mollusks) 230-208 MYA
– Trilobites (similar to horseshoe crab) 400 MYA
Radioactive Dating
• Determining the approximate age of fossils in
order to determine the organisms and absolute
dates they were living. This is often called
Radiometric Dating.
• Atomic Number – protons in nucleus
(corresponding electrons)
• All atoms of an element have the same atomic
number, but the number of neutrons can vary.
When they vary, the atom is called an “isotope”.
• Radioactive Dating – calculate the age of a
sample based on the amount of remaining
radioactive isotope it contains.
– Half-life: The length of time required for half of
the radioactive atoms in a sample to decay.
– For Rocks, there is Potassium-Argon or
Uranium-Lead dating.
Specimen Dating
All living things contain Carbon-14, a
radioactive isotope of Carbon-12. So for
once living organisms, (versus rocks) we
use Carbon Dating.
Because it is unstable, it wants to shed the
extra neutrons to stabilize.
This is called “radioactive decay”, and by
studying it, we can determine the how
many thousands of years ago something
was once living.
For instance, when an organism dies, the C12 stays constant, but the isotope, C-14,
(Carbon-14 half life is 5,730 years)
Carbon dating is really only good until
about 60,000 yrs old because then there is
not enough C-14 to get a good reading.
Isotopes decay at constant rates.
A half-life is the amount of time it takes for an unstable isotope to decay to one-half
its original amount.
Radiometric data allows scientists to then calculate the age of a specimen.
Your Turn!
Half-life of Carbon-14: 5,730 years
Uranium-238: 4,500,000,000 years
1. You have 16 grams of an isotope with a known half life of 20 years. How much
would we have in 80 years?
2. If we found in a specimen that had 75% degraded Carbon-14 (with only 25% left)
then how old is this specimen?
3. Why is there controversy over the disposal of nuclear waste (U-238)? Why do we
need to be concerned? (Think about the duration of its half -life!)